This invention relates to a zoom lens system for a camera, and more particulartry to a zoom lens system which is capable of effecting precise compensation in a zooming operation.
The typical variable focal length or zoom lens system has a variator lens and a compensator lens. The variator lens serves to vary a magnification factor of image of an object to be photographed, while the compensator lens serves to compensate the displacement of an image point due to the zooming effect of the variator lens so as to bring the object always into focus on a film. For this purpose, the variator and compensator lenses are disposed in camming engagement with a cam tube with grooves which is rotatable for mutually simultaneously effecting predetermined cammed axial movements of the variator and compensator lenses for focusing in a normal zoom range between a telephoto condition and a wide angle condition. The zoom lens system in the prior art thus includes a cam tube which must be provided with precisely formed grooves for guiding the variator and compensator lenses along the optical axis in response to operation of a zooming sleeve for precise compensation in the zooming operation. By reason of precision, it is very difficult to manufacture the cam tube made of plastics in an attempt to produce an economical zoom lens.
On the other hand, a proposal has recently been made in which cam surfaces are provided against which the variator and compensator lenses are resiliently urged by a spring supported at one end thereof on the cam tube and at the other end on a stationary frame. The variator and compensator lenses are thus controlled for axial relative movement according to the cam surfaces rather than according to the grooves or slots on the cam tube. In such an arrangement, the cam tube may advantageously be made of plastics, but a zooming torque for the cam tube fluctuates and the cam tube sometimes undergoes an oscillation because the spring changes in urging force in response to rotation of the cam tube.